With the ever increasing microminiaturization of semiconductor integrated circuits, and thus increasing lateral semiconductor device densities in integrated circuits, in recent years a major portion of the integrated circuit art has been moving in the direction of utilizing various forms of lateral dielectric isolation in order to laterally electrically isolate the densely packed devices from each other.
One approach for forming lateral dielectric isolation which has been increasingly utilized in the art involves the formation of recessed silicon dioxide lateral isolation regions, usually in the epitaxial layer where the semiconductor devices are to be formed, through the expedient of first selectively etching a pattern of recesses in the layer of silicon, and then thermally oxidizing the silicon in the recesses with appropriate oxidation blocking masks, e.g., silicon nitride masks, to form recessed or inset regions of silicon dioxide which provide the lateral electrical isolation. Representative of the prior art teaching in this area are U.S. Pat. No. 3,648,125 and an article entitled, "Locos Devices", E. Kooi et al, Philips Research Report 26, pp. 166-180 (1971).
While this approach has provided good lateral dielectric isolation, it has encountered some problems. Originally, the art applied the silicon nitride masks directly onto the silicon substrates. This gave rise to problems associated with high stresses created on the underlying silicon substrate by the silicon nitride-silicon interface. Such stresses were found in many cases to produce dislocations in the silicon substrate which appear to result in undesirable leakage current pipes and otherwise adversely affect the electrical characterisitics of the interface. In order to minimize such interface stresses with silicon nitride layers, it has become the practice in the art to form a thin layer of silicon dioxide between the silicon substrate and the silicon nitride layer. During such thermal oxidation, there is a substantial additoinal lateral penetration of silicon oxide from the thermal oxidation beneath the silicon nitride. This lateral penetration is greatest at the mask-substrate interface to provide a laterally sloping structure known and recognized in the prior art as the undesirable "bird's beak".
The publications, "Local Oxidation of Silicon; New Technological Aspects," by J. A. Appels et al, Philips Research Report 26, pp. 157-165, June 1971, and "Selective Oxidation of Silicon and Its Device Application", E. Kooi et al, Semiconductor Silicon 1973, published by the Electrochemical Society, Edited by H. R. Huff and R. R. Burgess, pp. 860-879, are representative of the recognition in the prior art of the "bird's beak" problems associted with silicon dioxide-silicon nitride composite masks, particularly when used in the formation of recessed silicon dioxide by thermal oxidation.
Another approach to the formation of dielectric lateral isolation in integrated circuits is the "etch and refill" technique. By this approach, recesses are etched in the desired isolation pattern in the substrate, and dielectric materials such as silicon dioxide which can be formed by chemical vapor deposition are deposited over the surface of the substrate, thus filling the recesses as well as depositing over the unrecessed portions of the substrate to the same height as in the recesses. This produces an undesirable corrugated effect of peaks and valleys. It is, of course, recognized in the art that in order to further utilize the structure in subsequent integrated circuit fabrication, planarization is necessary, i.e., the dielectric material such as silicon dioxide must be removed from the unrecessed portion of the substrate while being permitted to remain in the substrate recesses. One of the recognized shortcomings of such prior art "etch and refill" techniques has been the difficulty in achieving such planarization.
Another difficulty encountered with such "etch and refill" techniques has been ensuring that the deposited dielectric material completely fills the recess, particularly in the case of relatively deep and narrow recesses.
Finally, irrespective of the method utilized to form the recessed dielectric isolation, there have been indications that when the recessed dielectric material is silicon dioxide, there is a tendency for inversion to take place along the interface of any recessed silicon dioxide and a P-type silicon substrate due to a positive charge inherently encountered in silicon dioxide
Also in bipolar devices where P-type regions such as the base abut the recessed silicon oxide, there is a tendency for such an inversion to occur, creating a leakage path across the region.